Construction and Evolution of the Galaxy Where do the dwarf galaxies fit in? Matthew Shetrone February 26, 2009
Dec 22, 2015
Construction and Evolution of the Galaxy
Where do the dwarf galaxies fit in?Matthew Shetrone
February 26, 2009
Why we care about dwarf galaxies
<- CDM vs. HDM ->
CDM wins and suggests that smallest scales form first and build larger galaxies.
What are these smallest scales that form first?
Lots of dwarf satellites!
Biggest GC
M15 Mv=-9 M=1e6
NGC6388Mv=-10M=1.5e6
Fornax Mv=-13 M=68e6 Sculptor Mv=-11 M=6e6
Carina Mv=-9 M=13e6 Sextans Mv=-9 M=19e6
Where are abundances done on these dwarf galaxies• Keck
– HIRES (35k) published (V=18 for S/N 30 in 4 hours).– DEIMOS (6k) published (V=19 For S/N 40 in 1 hour).
• Subaru– HDS (45k) published (V=17 for S/N 55 in 2.5 hours).
• VLT – UVES published (45k) limited to V=19 for S/N 30 in 5 hours.– FLAMES-UVES (45k) limited to V=18 for S/N 30 in 5 hours.– FLAMES-Giraffe published (20k) limited to V=19 for S/N 30 in 7 hours.
• Magellan Landon Clay– Mike (20k) published (V=17 for S/N 40 in 1.5 hours).
• HET – HRS (15k) limited to V=18 for S/N 30 in 3 hours.
• Gemini
– Phoenix (50k) published (K=13 for S/N 50 in 2 hours).
Modeling the ResultsLanfranchi & Matteucci 2004
Standard (but sophisticated) chemical evolution models including metal-rich winds, super novae and AGB yields, and SFH from HST photometry.
When tuned to match the Milky Way these models can match the dSph galaxies including the alpha, iron peak and rare earth elements.
They find slower chemical evolution and high wind efficiency.
The Ultra Faint Dwarfs
Faint GC
M 71
Mv=-5.6
Pal 13
Mv=-3.5
Old OC
NGC 7789
Mv=-4.7
NGC 188
Mv=-3.5GC and OC have sizes < 10 pc
Observing large samples at lower resolution
For many years people have been measuring the CaT lines and determining a metallicity analog.
Shetrone et al. 2009 have been able to analyze the weaker Fe, Ti and Mg lines in these spectra to get individual abundances for the Leo II dSph: errors on individual stars are large but the mean trends are reliable.
Evan Kirby has been developing a technique for determining alpha and Fe for DEIMOS-Keck spectra in a large number of dSph galaxies. This technique is growing in sophistication and will soon measure individual abundances.
dSph and the MW Halo
We have already begun the comparison with the Milky Way by comparing the chemical abundance patterns.
The result?
The halo could/might look like the most metal-poor dSph stars but definitely does not look like the more metal-rich (relatively) dSph stars.
Who cares? The halo clearly formed fairly quickly before such stars would have formed and thus everything is consistent, right?
Metallicity Distribution Function
Helmi et al. 2006 find a difference between halo and dSph, while Schoerck et al. 2008 do not.
Interpretation seems to depend upon where you normalize the distribution function.
What is the goal?
A question of where to normalize
An Nbody/Tree SPH simulation of Sextans dwarf galaxy. (Revaz et al. 2009)
Metallicity Distribution Function
Interpretation seems to depend upon where you normalize the distribution function.
What is the goal? To model the early fast evolution of metallicity (ie. SN II from first and second generations).
Should avoid stars with Type Ia contributions for [Fe/H] plot OR plot again [alpha/H].
dSph and the MW Halo
Very few dSph have full space motions, but those that do don’t come within 10 kpc to the Galactic center or the solar neighborhood:
Canis Major: Rperi = 11 kpc Dinescu et al. 2005Carina: Rperi = 20 kpc Piatek et al. 2003Fornax: Rperi = 138 kpc Dinescu et al. 2004Sculptor: Rperi = 120 kpc Dinescu et al. 2004Ursa Minor: Rperi = 40 kpc Piatek et al. 2005
Leo II has little to no tidal interaction with MW (Siegel et al. 2008)Even LMC and SMC MAY be on first approach (Belsa et al. 2007)
Sagittarius is the exception.
The Local Inner vs. Outer
Abundance Results
There is a larger dispersion for outer halo abundances.
Roederer 2009
Some Science Results• Results discussed on the dSph
– Slower evolution for dwarf galaxies where winds important– Sub-solar [alpha/Fe] in the most metal-rich stars.– Origin of the most metal-poor galactic halo stars may be the least luminous
“galaxies”.• Results not discussed on the dSph
– Constraints on the origins for Mn, Cu, Al and Na.– Evidence for different origin and production rates of the light and heavy alpha
elements, e.g. Ca up while Mg down.– Multiple populations within the dwarfs (younger/metal-rich centrally
concentrated).• What Surveys may hold for dSph and the Milky Way
– Distribution of Ultra Faint Dwarfs over the sky– In situ samples for outer halo– A small sample of confirmed escapee metal-rich dSph star in the halo.